Potential Evapotranspiration (PET)

Potential evapotranspiration, or PET, represents the combined loss of water through:  1) the plant’s process of transpiration via its vascular system, and 2) evaporation of water from the earth’s surface.  Both are influenced by temperature, humidity, sunlight, and wind.  PET values indicate the amount of water that has been lost, and thus needs to be replaced, through irrigation and/or rainfall.

PET data is used to calculate the water requirements of crops and landscape plants, and adjust irrigation schedules in a way that encourages efficient water use, reduces waste, and saves money.  Reduction in over-watering also reduces pollution from fertilizer and chemical run-off into waterways.  With projected increases in population and water demand, water conservation is rapidly becoming an issue in all parts of Texas, not just regions of low rainfall.  Conservation methods must be implemented now, as one method of addressing future demands.

Tip: PET values will be lower on cloudy, cool, or rainy days compared to sunny, warm days with low humidity.  Wind will increase PET values, because evapotranspiration rates are higher.

Example: Add up the PET values for the past seven days.  If you water your yard once a week, this is the amount of water you need to put out when irrigating.  If it has rained during those seven days, subtract the total amount of rainfall from the PET total for the same time period.  This will be your correct PET value, adjusted for rainfall.

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